The pre-segmentation period of Aurelia strobilation will be investigated at the cytological level. Organisms induced to strobilate by thyroxine will be explored with various techniques to determine cytological responses to the hormone at various time periods prior to segmentation (first morphological sign of strobilation in these Aurelia). The general morphology of the cells of the Aurelia will be recorded at the light microscope and ultrastuctural levels. Special procedures will be used to explore possible nerve proliferation and/or increased neurosecretory activity during the early strobilation time period. Emphasis will be placed on the possible role of thyroxine in initiating extensive mitotic activity in the organisms. Correlations will be made between the "simple" metamorphic process of Aurelia strobilation and other thyroxine-influenced developmental systems of higher organisms. Information from these studies will be used to advance knowledge regarding regulatory mechanisms involved in the control of biological growth and differentiation and to improve our understanding of the action of thyroxine on developmental processes at the cellular level.